1. Field of the Invention
The present invention relates to a waveform generating apparatus and a waveform generating program suitable for use in an electronic musical instrument.
2. Description of the Related Art
Conventionally, a harmonic synthesis method and an additive synthesis method are known. The harmonic synthesis method synthesizes a waveform having an arbitrary harmonic structure by overlapping a fundamental harmonic and harmonic components of the fundamental harmonic. The additive synthesis method synthesizes a desired waveform by overlapping a plurality of elementary waveforms, including various waveform cycles and waveform shapes. As an example of a waveform generating apparatus configured based on the latter additive synthesis method, an apparatus is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2000-181459. The disclosed apparatus stores a plurality of elementary waveforms, including various waveform cycles and waveform shapes, in a memory in advance, and reads elementary waveforms required to synthesize a desired waveform from the memory. Then, the apparatus performs a predetermined weight multiplication on each of the read elementary waveforms, adds the weight multiplication results, and forms one waveform.
Waveform generation using the additive synthesis method has problems such as the following. For example, it is known that, when two correlated waveforms having the same pitch are overlapped, a cancellation of frequency components occurs depending on a phase difference between the two waveforms. As a result, a tone that sounds as though it has been passed through a comb filter is generated. To consistently generate such tone modifications in the same manner, a constant amount of phase difference must always be maintained between the waveforms to be overlapped. To always maintain the constant amount of phase difference between the waveforms to be overlapped, waveform starting points are required to match. Furthermore, pitch modulation by, for example, low-frequency oscillation (LFO) and pitch-bend must always be synchronized with a plurality of sound production channels and continuously provide a same modulation width.
In addition, depending on the type of synthesized waveform, synthesis of the waveform used in the additive synthesis can be started during a sound production of an entire synthesized waveform or ended during the sound production of the entire synthesized waveform.
However, it is extremely difficult to synchronize a new additive waveform with a phase of a synthesized waveform that is already being sounded. Therefore, regardless of whether the waveform will be sounded, all waveforms that may possibly be used in the additive synthesis are required to be synchronously reproduced from the start to the end of the sound production of the entire synthesized waveform. As a result, the sound production channel (oscillator) is needlessly wasted.